expensive to post parcels home to the UK?
depends what is in it.
If I were sending two or three dress shirts or a couple of sweaters I would put them in a large envelope and send them as a letter. You are allowed to do this, and they will go by air mail.
I wouldn't say that Polish post is expensive, but you can make it cheaper for yourself, if you know the shortcuts.
Always make sure that whatever you send is registered one way or the other.
This has been dealt with on other threads.